In recent years there has been increasing interest in ethanol, especially aqueous ethanol, as a feed stock for producing ethylene by catalytic dehydration. The basic dehydration reation is well known, and a number of different dehydration catalysts have been proposed and used for this purpose. Commercially, it appears that the preferred catalysts have been alumina or alumina-silica, such as alumina and magnesia deposited on a porous silica carrier. (See Haggin, C & EN, May 18, 1981, pages 52-54.) Bauxite activated with phosphoric acid (H.sub.3 PO.sub.4) has been proposed. Chem. Abst., 91, 12305 (1979). A catalyst called Syndol, is stated to be used in the commercial production of ethylene from ethanol (N. K. Kochar, R. Merims, and A. S. Padia, Chem. Eng. Progr., June, 1981, 77, 66-70). The structure of the catalyst is not divulged however. Ethylene from this source contains about 2% of mixed hydrocarbons as impurities. Recently, experiments have been reported using polyphosphoric acid for the liquid phase conversion of ethanol to ethylene, the reaction being conducted on a batch basis with the polyphosphoric acid dissolved in the ethanol starting material. Pearson et al, Ind. Eng. Chem. Prod. Res. Dev., 19, 245-250 (1980).
During the unpublished experimental work leading to the present invention, polyphosphoric acid was applied to a porous granular support (viz. charcoal) and used as a vapor phase catalyst for the conversion of aqueous ethanol to ethylene. The catalyst was initially effective, but its conversion efficiency declined with increasing runs. It was determined that the polyphosphoric acid was being removed from the catalyst granules by the action of the water vapor (steam) present in the catalyst bed, water vapor being present in the aqueous ethanol feed stock and also being formed in the dehydration reaction. The use of phosphoric acid on a catalyst support is subject to this same defect, that is, it will be progressively removed from the granules of the catalyst bed where the vapor phase reaction is being carried out in the presence of large amounts of water vapor. The present invention provides a means for markedly reducing catalyst loss, and provides the art with a new and highly effective catalyst for the vapor phase dehydration of ethanol to ethylene as well as for other purposes in which water vapor is present in the catalyst bed.